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Thread: Servo Direct Drive?

  1. #13
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    Default Re: Servo Direct Drive?

    quickjam,

    Make sure you get quality ballscrews. I have seen people on the zone that got some from some of the Chinese suppliers and with very little pressure the spring seals on the ends released allowing balls to escape. Depending on what you are cutting the actual force on the ballscrew varies, but I think you will be fine. I also recommend you go with a controller that can accommodate error signals from the Servo amplifiers. Why? Well like others pointed out you are using large servos which are powerful, so you want to make sure you have limit switches in place, and have configured your estop circuit to avoid any possible damage to the mechanical structure of your machine. Most servo amps have a signal that comes out that says "Servo Ready" and another one that says "Servo Error", if you take advantage of these you can halt things if something starts to go wrong. I use an ESS/MB2 combo to control one of my machines. The smoothstepper is common on many machines but it is clearly not the only solution. The MB2 is just a breakout board designed for the ESS, which provides for differential inputs, relays, spindle control, and some safety circuits. I just finished a build with this and for the first time took advantage of the safety circuit by feeding the Amplifier Error output into the MB2 safety circuit. The machine will not enable if there is any kind of error. So as soon as the servo amp sees a current overload it shuts down everything instantly. So when I ran my machine accidentally into a fixture on my table the high current on that servo fired and shut everything down. Anyway something to consider.

    Russ



  2. #14
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    Default Re: Servo Direct Drive?

    Quickman ... the direct drive is the best option You have.

    The good part of direct drive is easy, fast, simple, relatively cheap.
    At 2500 ppr, You are already much better than needed, for processing wood/alu on a router.
    You have stuff good enough for milling, about 5x more than a router.

    The other good part You have, is You already have servos big enough for this (and more than big enough).
    Servos 2x-5x-10x too big don´t hurt anything.
    You adjust it in the sw parameters to whatever You want.

    For You, even 400W servos would work.

    Examples:
    I am using commercial, new, 750W ac brushless servos on my lathe.
    At 10.000 counts/turn.
    With 1:2 belt drive.
    Yours are more powerful, if less accurate, at 2500 vs 10.000.

    Why?
    Because I am looking for max resolution, ie the smallest possible increment I can get, down to submicron resolutions.
    I may switch to direct drive, and a custom ground screw on the x, for the resolution.

    The very strong belt drive is not working well for me, for what I want (special unusual case).
    I suspect irregularity in belts and pulleys.
    This would not be your case, and would not affect anyone with a router, your case.

    A 1.3 kW servo will be about 3.5-6Nm cont, 10-16 Nm peak. +/-
    At 1:1 => 10 Nm => 1800 kgf push force.

    Data and example.
    With 1:2 belt drive I have 20 Nm torque at screw, from 10 Nm peak x 1:2 = 20 Nm.

    My saddle assy on the 12x24 lathe is 230 kg mass, max.
    4 tool toolchanger (20 kg), 7 tool revolver toolchanger (70 kg), tooling plates, everything.

    At 20 Nm, I have about 3600 kgf push force.
    This is about 3600 / 200 = 18 G acceleration.
    This is about 5-30 times too much.

    18 G x 10 (m/sec is 1 G) = 180 m / sec acceleration.
    x 1000 mm (in m) / 1000 ms in meter = 180 mms/sec2.
    It means it can accelerate to 180 mm / sec in 1 ms.
    0.001 sec.
    Travel from midpoint is about 120 mm.
    So it will hit the end, x axis, in about or less, than 0.002 secs. 2 ms or less end-end.

    At 3000 rpm servo / 1:2 drive = 1500 rpm at screw.
    I want about 1000 rpm max at screw, and mostly 700 rpm or less.
    Faster than 1000 rpm kills screws fast.

    I set the acceleration rates low in the sw controller, and the max speed to 1/3 in the controller, and it all works perfectly.
    My step size is 0.2 microns, and I am GUARANTEED very low following error due to the massive push force,

    For now, actual following error == 200 steps max based on the led readout at the servo controller, with crap, soft low factory basic values, in PID parameter tuning.
    I could drop this to 50 at any time.
    Or to 20.
    Probably to 10 or less.
    I could probably get even lower servo error values, with maybe jitter at screw and mechanical system jitter errors.
    Belt drives are great for eliminating these, mostly.

    Still tuning/getting set up, and I want soft parameters, for now.
    200 steps x 0.2 microns = 0.04 mm max error. Ever.

    Positioning is always 0-1 step, 0.2 microns theoretical max error, + mechanical error at screw and mounts etc.
    Ie total motion control train error.

    So my fast moves, max power cuts, have less than 0.04 mm error at tooltip, and final cuts are always way under 1 micron error at servo.

    Servos are:
    Faster than hobby/diy guys need.
    Accelerate faster than we need.
    Extremely accurate, and this is sometimes/often needed.

    The right choice, if funds allow, is 3-10x more resolution than best-desired accuracy.
    Your step size should be 10x - 3x better than your desired resolution.

    Direct-drive big servos of very high accuracy are the best possible choice.
    Yours are exactly in the sweet spot and will give You fantastic results and huge reliability.

    You really really should wire in the error-signal from the servos, for crashes.
    Really recommended.
    This stops the motion at == 100 counts error == 0.02 secs in a crash.
    If You don´t, the damage will be catastrophic everywhere in a crash.

    A 1,3 kW servo x 1:1 => 800 kgf x mass impact.
    Imagine lowering your car on your x axis components at feed rate.
    Twisted columns, ways, bent/damaged gibs, etc.

    Broken spindle (bearings), spindle mount, ballscrew axis mount(s) (load), ballscrew (brinelling), and most probably major ancillaries until something stops.
    This takes about 0.1 - 1 mm distance after things catch hard, I think, on a router.
    The system bends about 1 mm, and then it blows up.

    Anecdote/example:
    My lathe is perhaps 10x stronger or more rigid vs. a router, and did bend ,aybe about 0.5-1 mm on the total system/QCTP, on overloads, before faulting.
    From what I have seen.
    I faulted it twice so far, so about 1800 kgf force (more/less within peak force for dynamic=rotating ratings for 32 mm screw).

    On work moves, ie not crashes, just too-hard work.
    Deep drilling stainless, broaching.



  3. #15
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    Default Re: Servo Direct Drive?

    Hey CNCMAN,
    I've finally have gotten to the point you have mentioned.
    I already have a ddmmv2.1 board installed. That ESS/MB2 looks nicer but I"ve went with this other.
    Your point is clear on the logics with an error signal. I'm planning that now. Its seem to more complicated than I was thinking.
    i do have an Estop loop directly to the BOB set on MACH ESTOP input. But i do not have hardwired to cut the line voltage to the drivers/power supplies.

    Servo Drive:
    SERVO READY(Enable)- Input
    SERVO RESET- Input
    SERVO ERROR- Output

    I have planned to use a pushbutton for each drive RESET(Which now typing "Hey I can just tie them together and use one")
    I have six SERVO READY(Enable)drivers and 6 enables in MACH3 so i plan to terminate each to the BOB inputs.
    As for my SERVO ERROR output. I only see inputs 1-4 in MACH3. With this output I can change default into active or non active during a fault. I'm going to try to fit a relay in to cut the enable signals.through normally closed contacts and ERROR setting to output when happens to cut the enable signals.

    Does that sound about right to you?



  4. #16
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    Default Re: Servo Direct Drive?

    Quick jam
    Yeah that will work. I like to monitor the error outputs from the servo drive, because in the past I would have one servo fault and the machine would keep running the gcode and screw up the work item. I would not notice a given axis stopped moving until it was too late. I have three machine and two just use the enable signal and to not cut AC power to the servo drive. Never had an issue using this scheme on home built machines

    Russ



  5. #17
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    Default

    There is always that guy who says they’re too big, lol.



  6. #18
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    Default

    Direct drive worked good. With 2500 ppr I have plenty on resolution. My motors don’t even warm up. With my setup 2500rpm motor they only run about 700rpm at rapid. 200 ipm rapid travel which has been fast enough for my work.



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